1. Field of the Invention
The present invention relates to an improved positive charge pump, particularly for the integration in CMOS integrated circuits such as non-volatile memory devices.
2. Discussion of the Related Art
As known, a positive charge pump is a circuit suitable for generating, starting from a positive voltage having a predetermined value, a higher voltage.
Conventionally, a positive charge pump includes a plurality of stages connected in series between an input terminal of the charge pump connected to a positive voltage supply and an output terminal of the charge pump. In the simplest case, each stage substantially includes a diode-connected N-channel MOSFET and a capacitor having one plate connected to the source electrode of the MOSFET and another plate driven by a respective digital signal periodically varying between ground and the voltage supply.
In steady-state operation, the voltage gain of each stage is equal to the value of the voltage supply minus the threshold voltage of the N-channel MOSFET.
More sophisticated charge pumps have stages wherein the diode-connected MOSFET is replaced by a pass-transistor, and comprise circuits for boosting the gate voltage of the pass transistor. In this way, as long as the body effect on the threshold voltage of the pass-transistors is not very high, it is possbile to partially compensate the increase in the threshold voltage.
The main problem of these circuits resides in the fact that, due to the body effect, the threshold voltage of the N-channel MOSFETs becomes higher and higher moving from the stages proximate to the input terminal to the stages proximate to the output terminal of the charge pump. This causes a reduction in the voltage gain of the stages, and makes it necessary to increase the number of stages in order to generate a given output voltage.
A greater number of stages requires more area in the chip, and higher power consumption. Furthermore, it is not possible to generate output voltages higher than a given value, because when the threshold voltage (with body effect) of the N-channel MOSFETs becomes higher than the voltage supply, the addition of further stages is completely ineffective. Even the more sophisticated solution becomes less and less effective moving from the input terminal to the output terminal of the charge pump, and the final stages have poor efficiency.
In view of the state of the art described, it is an object of the present invention to provide an improved positive charge pump which is not affected by the above-mentioned problem.